Applicability of glass fiber as a support material for generation of gaseous standard mixtures using thermal decomposition of the surface compound. Calibration of analytical equipment

The use of glass fiber as a support material for a surface compound serving to generate gaseous standard mixtures of ethene is described. The technique is based on the process of thermal decomposition of the surface compound in a desorber connected on‐line via a multi‐port valve to the calibrated device. The surface compound undergoes thermal decomposition at 245°C, yielding known amounts of ethene. The method enables on‐line preparation of a standard mixture immediately before the calibration step. Consequently, it can be also applied for the generation of standard mixtures containing volatile, malodorous, unstable, and toxic compounds.     

New procedure of silica gel surface modification preparation of gaseous standard mixtures for calibration purposes

The new type of silica gel surface modification with using the trimethylamine as a reagent is described. The samples of chemically modified silica gel have been used for generation of gaseous standard mixtures (methyl chloride as a measurand) using the technique of thermal decomposition of the surface compound. The main aim of the research was to check the suitability of the new type of silica gel surface modification for obtaining methyl chloride as a measurand of gaseous standard mixture. The gaseous standard mixture obtained with using this technique was used for calibration of a thermal desorber-gas chromatography-flame ionization detector (TD-GC-FID) system. The homogeneity of coverage of silica gel surface with the immobilized compound has been evaluated. The full uncertainty budget of determination of liberated amount of methyl chloride has been calculated. The average amount of methyl chloride liberated from the unit sample of chemically modified silica gel is 3.59 +/- 0.13 mg g(-1). The influence of the modification way on the amount of liberated analyte has also been determined.     

Thermal decomposition of immobilized compounds for the generation of gaseous standard mixtures containing ammonia and amines

The thermal decomposition of phthalamic acid derivatives chemically bonded to the surface of silica gel was examined and utilized for the generation of single-component gaseous standard mixtures of ammonia, methylamine, diethylamine and triethylamine. The conditions of the thermal decomposition (temperature, time, diluent gas flow rate) were optimized to ensure complete liberation and rapid elution of the compounds from the bed of modified silica gel. The total amounts of these four compounds that can be released from unit mass of the modified silica gel are in the range of several mg.     

Thermal decomposition of hydrotalcite-like compounds studied by a novel tapered element oscillating microbalance (TEOM): Comparison with TGA and DTA

For the first time, we report on the application of a tapered element oscillating microbalance (TEOM) as a novel technique to investigate the thermal decomposition of hydrotalcite-like compounds (HTlcs) in air. Experiments were performed in the temperature range of 323-973 K with Mg-Al, Ni-Al, and Co-Al-HTlcs. The TEOM technique measures mass changes based on inertial forces, presenting important advantages over conventional thermogravimetric analyzers, such as the very rapid time response and the well-defined flow pattern. In general terms, excellent agreement between TEOM, TGA, and DTA techniques during HTlc decomposition was obtained. Interestingly, transition temperatures in the TEOM were lower than in TGA and DTA, particularly for removal of interlayer water but also for dehydroxylation of the brucite-like layers and decarbonation. This was attributed to the flow-through operation in the tapered element of the TEOM as compared to the recognized gas stagnancy and bypass in sample crucibles of conventional thermogravimetric analyzers. Our results conclude that the TEOM technique is suitable for temperature-programmed studies. However, due to its operation principle, blank runs are required in contrast to the more automatic operation in commercial thermogravimetric units. Besides, a careful sample loading and packing in the micro-reactor is essential for reproducible results.     

Influence of surface modification on protein retention in ion-exchange chromatography: Evaluation using different retention models

A large number of different stationary phases for ion-exchange chromatography (IEC) from different manufacturers are available, which vary significantly in a number of chemical and physical properties. As a consequence, binding mechanisms may be different as well. In the work reported here, the retention data of model proteins (α-lactalbumin, β-lactoglobulin A, bovine serum albumin and alcohol dehydrogenase) were determined for three anion-exchange adsorbents based on synthetic copolymer beads with differences in the functional group chemistry. Fractogel EMD DEAE and Fractoprep DEAE consist of functional groups bound to the surface via “tentacles”, ToyopearlDEAE by a short linker. Three models which describe chromatographic retention were used to analyse the characteristic parameters of the protein/stationary-phase interactions. The number of electrostatic interaction between the stationary phase and the model proteins, the protein specific surface charge densities and the interacting surface of the proteins with the adsorptive layer of the chromatographic media depend on the surface modification as well as on the molecular mass of the model proteins. In general, protein retention of the model proteins on the weak anion exchangers was found to be greater if the stationary phase carries tentacles and protein mass is above 60 kDa.     

An unsteady state method for the measurement of polymer thermal diffusivity: I. Development of a cell

In this work the development of a cell able to determine the thermal diffusivity of polymers in their fused or solid state, using an unsteady state technique, is presented. In this case a step down perturbation method was used because of its easy experimental setup. For each experimental run a temperature step down perturbation of 10 °C was applied, and the temperature evolution with time and position were registered and saved. Thermal diffusivities were then determined by regression using those data and a simple analytical model. The polymers used to test the cell were high density polyethylene and polypropylene (PP). For the first polymer, the thermal diffusivity was found to be 2.05×10⁻⁷ m²/s, which compare satisfactorily with a value of 1.97×10⁻⁷ m²/s calculated from reported values of density, heat capacity and heat conductivity. For the PP the value was found to be 7.08×10⁻⁷ m²/s with a positive deviation of 5.05% when compared with a computed value of 6.74×10⁻⁷ m²/s. Taking into account experimental errors, and the natural variations between different stocks of materials, the observed differences are acceptable.     

Intercalation compounds of γ-zirconium and γ-titanium phosphates: Kinetic study of dehydration and decomposition processes for 2,9-dimethyl-1,10-phenanthroline and its intercalated copper complex materials

γ-Zirconium and γ-titanium phosphates containing organic diamine 2,9-dimethyl-1,10-phenanthroline and its in situ formed copper complex were studied by thermal analysis and physical measurements. All the derived materials show a layered structure and their interlayer distance is increased with respect to that of their precursors. Melting, simultaneous vaporization and oxidation, as well as a decomposition process take place in the pure diamine. After dehydration, all the intercalation materials undergo a two-step decomposition. The presence of the formed copper complex between the layers of the two ion-exchangers considered shows a destabilizing effect with respect to the intercalated diamine materials, resulting in a decrease of the decomposition temperatures and the activation energy of decomposition. The application of the isoconversional Ozawa-Flynn-Wall method substantially confirms the obtained results.     

Polymer/montmorillonite nanocomposites with improved thermal properties: Part I. Factors influencing thermal stability and mechanisms of thermal stability improvement

The results of recent research indicate that the introduction of layered silicate - montmorillonite - into polymer matrix results in increase of thermal stability of a number of polymer nanocomposites. Due to characteristic structure of layers in polymer matrix and nanoscopic dimensions of filler particles, several effects have been observed that can explain the changes in thermal properties. The level of surface activity may be directly influenced by the mechanical interfacial adhesion or thermal stability of organic compound used to modify montmorillonite. Thus, increasing the thermal stability of montmorillonite and resultant nanocomposites is one of the key points in the successful technical application of polymer-clay nanocomposites on the industrial scale. Basing on most recent research, this work presents a detailed examination of factors influencing thermal stability, including the role of chemical constitution of organic modifier, composition and structure of nanocomposites, and mechanisms of improvement of thermal stability in polymer/montmorillonite nanocomposites.     

Liquid core waveguide-based optical spectrometry for field estimation of dissolved BTEX compounds in groundwater: A feasibility study

The utility of a liquid core waveguide (LCW) system for acting as a sentry monitor for compounds such as benzene, toluene, ethylbenzene, xylene (BTEX) in water was examined. A vapor-permeable LCW suitable for long path length absorbance spectroscopy based on a Teflon AF 2400 tube was fabricated. Multiwavelength spectroscopy in the near-UV was carried out using a fiber optic-based flashlamp-photodiode array (PDA) combination with hexane as the solvent in the waveguide core. Using multicomponent calibration, quantitation of benzene and toluene accurate to 6±5% could be conducted at sub-mg l⁻¹ levels in mixtures after a sampling period of 10 min.     

Quadricyclane—thermal cycloaddition to polyfluorinated carbonyl compounds: A simple synthesis of polyfluorinated 3-oxatricyclo[4.2.1.0]non-7-enes

Quadricyclane (1) readily undergoes [2+2+2] cycloaddition reactions with electron-deficient fluorinated carbonyl compounds to give polyfluorinated 3-oxatricyclo[4.2.1.0^2,5]non-7-enes in high yields. Hexfluoroacetone, trifluoroacetyl chloride, methyl trifluoropyruvate, α-(fluorosulfonyl)difluoroacetyl fluoride, and bis(trifluoromethyl)ketene all react rapidly with 1. Trifluoracetyl fluoride although less reactive, slowly interacts with 1 at ambient temperature. 1,1,1-Trifluoroacetone, trifluoroacetophenone, carbonyl fluoride, and CF₃C(O)OC₆F₅ require higher temperatures (60-90 °C) for reaction, and ethyl trifluoroacetate is unreactive at 90 °C. Heating 1 with the ethyl hemiacetal of trifluoroacetaldehyde gives the corresponding cycloadduct of CF₃C(O)H in 44% yield.The oxetane product from hexafluoroacetone is remarkably stable to both acids and bases, whereas the oxetanes with α-F or Cl leaving groups are sensitive to acid-catalyzed rearrangement.     

Mechanism of generation of volatile species by aqueous boranes: Towards the clarification of most controversial aspects

The state of knowledge of the mechanisms involved in the chemical generation of volatile species (CHG) arising from aqueous phase reaction of classical hydride forming elements, transition and noble metals with borane complexes (mainly NaBH₄), has been critically reviewed in the light of evidences and literature data published in the last fifty years. The mechanisms, which are necessary to describe the reactivity of CHG system, are essentially: (i) the mechanism of hydrolysis of borane complexes, (ii) the mechanism of formation of volatile species, (iii) the mechanism of liquid phase interference and (iv) the mechanism of action of additives. Only the mechanisms (i) and (ii) have reached a good degree of rationalization, whereas more experimental evidences are necessary for the mechanisms (iii) and (iv). A more general reaction model for analytical CHG can be drawn according to the present state of knowledge, which is valid for both classical hydride forming elements and transition and noble metals. It is based on the formation of analyte–borane complex (ABC) intermediates through which takes place the direct, stepwise transfer of hydrogen atoms from boron to analyte substrate ML_n, (M is a metal or semi-metal, L is a ligand). By this way the original analyte substrate is stepwise converted to hydrido metal complexes MH_xL_y, then to the final products (hydride, metal atoms, etc). The clarification of several controversial aspects and the ruling out of wrong concepts, among them the “nascent” hydrogen theory, can been achieved in the light of the present state of knowledge.     

Fast GC analysis of major volatile compounds in distilled alcoholic beverages: Optimisation of injection and chromatographic conditions

The principal secondary flavour compounds in distilled spirits can be successfully quantified by split injection to a 0.15 mm internal diameter (I.D.) capillary column. Initial conditions for split ratio, gas velocity, initial oven temperature and oven ramp rate are given by method translation from a similar method on a standard 0.25 mm internal diameter column with the same phase. These parameters were then investigated in an experimental design comprising a series of experiments in which the responses were the resolution of two critical peak pairs, the analysis time and the limit of quantification (LOQ) of the eight major compounds. The LOQ is the concentration corresponding to a signal 10 times greater than the noise. The experiments were replicated at two different concentration levels, which encompassed the natural levels of the compounds of interest found in distilled spirits. From the chemometric evaluation of these data, a validated model was constructed, which allowed the prediction of conditions for optimum chromatographic analysis. Three additional concentration levels were then added to the model to establish linearity, repeatability and sensitivity. Modern gas chromatographic hardware allows the use of these narrow-bore capillary columns for routine use without operational difficulties. Major advantages are a substantial decrease in analysis time allowing high throughput processing of samples.     

Electro-adsorption of polyethyleneimine on the anion exchange membrane: Application to the nitrate removal from loaded solutions

Anion exchange membrane has been modified by fixation of polyethyleneimine. The modification has been carried out aiming to achieve a separation of anions according to their hydration radii and their sizes. The performances of the modified membranes were compared with the unmodified one by applying Donnan dialysis of nitrate ion using Cl⁻ in the strip compartment. The transfer of nitrate ions in the presence of chloride was enhanced by using immersion modified membrane. The presence of a thin layer of PEI on the strip side of the membrane improves Cl⁻ transference, by creating a chemical potential difference which increases the transfer.     

Estimation of interfacial behavior using the global phase diagram approach: I. Carbon dioxide-n-alkanes

This study looks for the first time at the possibility of predicting the interfacial tension in mixtures without preliminary resource to their experimental data. For this purpose the quantitative global phase diagram (klGPD)-based approach (GPDA), which needs only two or three key experimental points of one homologue for predicting the complete phase behavior in whole homologues series of binary systems, is combined with the gradient theory (GT) methodology. The resulting model is able to predict the data in satisfactory manner, although the increasing asymmetry between the compounds of the mixture probably affects the ability of GPDA to yield accurate predictions of phase equilibria and interface tension simultaneously.     

Phase Equilibrium in the System Ln-M-O: III. Ln=Gd at 1100°C

Phase equilibrium in the system Gd-Mn-O has been established at 1100°C while varying the partial pressure of oxygen between 0 and 13.00 in −log (P_O2/atm), and a phase diagram at 1100°C is presented as a Gd₂O₃-MnO-MnO₂ system. Under the experimental conditions, Gd₂O₃, MnO, Mn₃O₄, GdMnO₃, and GdMn₂O₅ phases are present at 1100°C, but Gd₂MnO₄, Mn₂O₃, and MnO₂ are not stable in the system. The substantial difference from the previously studied La-Mn-O and Nd-Mn-O systems lies in the fact that the LnMn₂O₅-type phase is stable under the present experimental conditions. A wide range of nonstoichiometry has been found in the GdMnO₃ phase coexisting with Gd₂O₃. X in GdMnO_3+X ranges from −0.03 at log P_O2=−9.47 to 0.05 at log P_O2=0. Nonstoichiometry is represented by an equation, N_O/N_GdMnO3=3.00×10⁻⁴(log P_O2)³+5.80×10⁻³(log P_O2)²+3.52×10⁻²(log P_O2)+0.0464, and the activities of components in solid solution are calculated from the equation. Similar to the case of LaMnO₃, GdMnO₃ seems to vary in composition between the Gd₂O₃-rich and Gd₂O₃-poor sides. Lattice constants of GdMnO₃ produced under different oxygen partial pressures and those of GdMn₂O₅ prepared in air were determined, along with spacings and relative intensities of GdMn₂O₅. Standard Gibbs energies of reactions shown in the system were calculated and compared with previously reported values.     

Sorbent-based sampling methods for volatile and semi-volatile organic compounds in air: Part 1: Sorbent-based air monitoring options

Sorbent tubes/traps are widely used in combination with gas chromatographic (GC) analytical methods to monitor the vapour-phase fraction of organic compounds in air. Target compounds range in volatility from acetylene and freons to phthalates and PCBs and include apolar, polar and reactive species. Airborne vapour concentrations will vary depending on the nature of the location, nearby pollution sources, weather conditions, etc. Levels can range from low percent concentrations in stack and vent emissions to low part per trillion (ppt) levels in ultra-clean outdoor locations. Hundreds, even thousands of different compounds may be present in any given atmosphere. GC is commonly used in combination with mass spectrometry (MS) detection especially for environmental monitoring or for screening uncharacterised workplace atmospheres. Given the complexity and variability of organic vapours in air, no one sampling approach suits every monitoring scenario. A variety of different sampling strategies and sorbent media have been developed to address specific applications. Key sorbent-based examples include: active (pumped) sampling onto tubes packed with one or more sorbents held at ambient temperature; diffusive (passive) sampling onto sorbent tubes/cartridges; on-line sampling of air/gas streams into cooled sorbent traps; and transfer of air samples from containers (canisters, Tedlar^® bags, etc.) into cooled sorbent focusing traps. Whichever sampling approach is selected, subsequent analysis almost always involves either solvent extraction or thermal desorption (TD) prior to GC(/MS) analysis. The overall performance of the air monitoring method will depend heavily on appropriate selection of key sampling and analytical parameters. This comprehensive review of air monitoring using sorbent tubes/traps is divided into 2 parts. (1) Sorbent-based air sampling option. (2) Sorbent selection and other aspects of optimizing sorbent-based air monitoring methods. The paper presents current state-of-the-art and recent developments in relevant areas such as sorbent research, sampler design, enhanced approaches to analytical quality assurance and on-tube derivatisation.     

Determination of endocrine disrupting compounds using temperature-dependent inclusion chromatography: I. Optimization of separation protocol

In the present work we optimised the separation of battery of key UV non-transparent low-molecular-mass compounds having possible endocrine disrupting compounds (EDCs) activity or which may be used as the endocrine effect biomarkers. Simple optimization strategy was based on strong temperature effect that is driven by electrostatic interactions between macrocyclic mobile phase additives like cyclodextrins and eluted components of interest under C18 stationary phase and acetonitrile/water mobile phase conditions. Particularly, the effect of temperature involving native β-cyclodextrin and its hydroxypropyl derivative to improve separation of number of natural (d-equilenin, equilin, estetrol, estriol, estrone, 17β-estradiol, 17α-hydroxyprogesterone, 20α-hydroxyprogesterone, cortisol, cortisone, progesterone, testosterone, tetrahydrocortisol and tetrahydrocortisone) and artificial steroids (ethynylestradiol, norgestrel isomers, medroxyprogesterone, mestranol, methyltestosterone, norethindrone, 17α-estradiol) as well as non-steroidal compounds (diethylstilbesterol, bisphenol A, 4-tert-butylphenol, dimethyl phthalate, dibutyl phthalate and dioctyl phthalate) was investigated. It has been found that successful isocratic separation of 27 chemicals can be achieved using acetonitrile/water eluents modified with β-cyclodextrin or hydroxypropyl-β-cyclodextrin at concentration of 10 mM and temperature of 47 °C. Separation protocol is simple, reliable, direct and non-radioactive and may be easily adapted for rapid separation and quantification of wide range of given steroids and related EDCs in environmental samples, particularly those that are characterised by unstable biological matrix and components of interest load.     

Toward a Unified Approach to the Crystal Chemistry of Aurivillius-Type Compounds.: I. The Structural Model

A generalized structural model of Aurivillius-type compounds is presented using a 4D superspace group analysis where Aurivillius structures are considered as cation-deficient perovskites with the general formula AB_1−xO₃. Being essentially composition independent, the model is valid for any Aurivillius-type compounds where x is the only composition-dependent parameter. The atomic domains representing the atoms in superspace are described by means of crenel functions. For any composition, the conventional space groups can be easily derived from a unique superspace group. This work is supported by a TEM investigation where the continuously variable character of the diffraction diagram indicates that the various stacking sequences can be interpreted in terms of a structural modulation over a common average structure.